The present invention relates, in general, to the field of safe operating circuits and methods for an output switching device. More particularly, the circuit and method of the present invention is of special utility in the protection of bipolar power output transistors of use, for example, in a voltage regulator.
Current through an output device, such as a vertical bipolar transistor, must be limited to operation within a parameter defined by a plot of such current with respect to the input-output differential voltage "V.sub.IN -V.sub.OUT ". Operation at a point beneath this current limit curve is defined as a safe operating area, or "SOA". For obvious reasons then, as this differential voltage increases, current must be concomitantly decreased to insure power constraints stay below this current limit. However, in order to most fully utilize the capability of an output device, operation should ideally track the current limit curve as closely as possible. Therefore, it is necessary to insure that the differential voltage be sensed and current limited in response thereto. An example for effectuating this purpose is disclosed in U.S. Pat. No. 4,319,181 issued on Mar. 9, 1982 to Robert S. Wrathall and assigned to Motorola, Inc., assignee of the present invention. In previous voltage regulators, a circuit was utilized which applied a step approximation to the SOA area of the current limit curve. In general, this approximation is poor and difficult to control. Moreover, this technique does not allow the use of a differential voltage, but rather only the maximum voltage applied to the output device.
Other voltage regulators utilize a relatively similar system having a zener controlled current in the current sense. However, the current sense is accomplished by developing a full V.sub.BE across the total current in the output device by means of emitter ballasting equaling a V.sub.BE drop.